1. Field of the Invention
The present invention is related to control systems for use with boilers and their associated burners. More specifically, the present invention is directed to foam compensating control systems that periodically turn the burner off to diminish foam and surging in the boiler and allow the true water level to be accurately determined.
2. Description of Related Art
Steam boilers require a minimum level of water to function properly and safely. Failure to maintain this minimum water level within the boiler can result in severe boiler damage and in some circumstances, can lead to boiler explosions.
A common method of monitoring the water level in a steam boiler is the electronic probe-type low water cut-off. This device shuts down the burner in the event that the water falls below the lowest safe level. Such a system uses an electronic probe that measures conductivity between two electrodes. One electrode protrudes directly from the probe into the boiler through a tapping provided by the boiler manufacturer. The second electrode is formed by the conductive boiler shell which is electrically connected to the mounting nut of the sensor.
The conductive boiler liquid, generally water, completes the circuit path between the electrode sensor in the boiler water and the boiler shell. When the liquid level drops below the sensor level, the conductivity between the two electrodes drops significantly. This reduction in conductivity is sensed by the control system and power is removed from the burner. Because the water level in a steam boiler can be very turbulent, a time delay is designed into probe-type controls to prevent short cycling of the burner circuit during momentary dips in the water level.
Under some operating conditions, common in poorly maintained boilers, the aforementioned probe-type cut-offs are limited in their ability to sense the true water level. If a steam boiler is not properly maintained through periodic cleaning, foam can be generated within the boiler which can be as conductive as water and consequently fall within the detection range of conventional probe-type low water cut-offs. For example, in extreme conditions, the water level in the boiler may drop below the sensor to an unsafe operating level. Foam on top of the boiler water, still at the sensor level, can complete the circuit path between the sensor and the boiler shell ground. Thus, although the true water level is below the sensor, the conductivity between the probe and the boiler shell remains high due to the presence of the foam and the burner remains on.
The problem described above is becoming more acute as the industry moves to smaller, more efficient boilers. Since these newer boilers have considerably lower water content than boilers manufactured years ago, contamination and foaming occur more quickly. In addition, since these boilers hold less water, they are more susceptible to low water conditions.
One approach to this problem has been the introduction of foam compensating burner control systems that cycle the burner on and off. The burner is turned on to produce steam, then turned off for an off-period to allow the foam to settle and any sloshing to diminish. Although the water level is constantly being monitored by the probe, the true water level can only be accurately determined when the foam has settled and the water has stopped moving. If the probe indicates that there is adequate water, the burner is then turned on again for an on-period before the boiling is stopped to accurately check the level again.
The on-period duration is selected to be long enough to produce steam, but not so long that there is any significant danger of boiling away water to below the safe level, given that the previous monitoring of the water level showed a safe level. The duration of the off-period is selected to be long enough to allow the foam to settle and water sloshing to substantially diminish so that an accurate detection of a safe water level can be made. An example of a control system of this type is shown in U.S. Pat. No. 5,739,504, assigned to the owner of the present invention: C. Cowles and Company, located in New Haven, Conn. Designs of the type shown in U.S. Pat. No. 5,739,504 have been quite successful, however, the present invention provides several advantageous improvements.
Boilers used with devices of the type described are provided with a demand control circuit to turn the burner on and off. When the boiler is first powered, the demand control circuit will turn the burner on to bring it to the preset temperature or pressure. When that point is reached, the demand control circuit turns the burner off until there is some demand or until heat loss causes the temperature or pressure to fall. This normal operation of the boiler means that the burner will be off for various reasons unrelated to the need to monitor water level. If the burner has been off for long enough to allow settling of the foam, then the probe will accurately determine if there is a low water level. In this situation, there is no need to turn off the burner again until after it has been on for the complete on-period. It is particularly undesirable to turn off the burner while the boiler system is attempting to meet demand, if it is unnecessary.
Prior foam compensating burner control system designs have run the described burner off/burner on cycle to monitor water level independently from the demand control circuit or any other controls on the on/off state of the burner. Thus, prior foam compensating burner controls might turn off the burner to check the water level only a short period of time after a demand has turned the burner on, even if the burner had previously been off for a substantial period of time during which a low water level would have been detected. It is desirable to eliminate this short cycling of the first burner on cycle if the burner had previously been off for long enough to let the foam settle.
Another difficulty with the constant independent cycling of prior foam compensating burner control system designs has occurred when the boiler fluid is at a low temperature, as occurs during initial heating, or during long periods of very heavy demand. If the boiler fluid is at a low temperature and is not boiling, there is no foam or sloshing and no reason to interrupt the burner to let the water settle. Some boilers use the boiler fluid to provide heat for potable hot water systems in a tankless coil design. These systems have a boiler fluid temperature probe that signals when the fluid temperature is low. It is desirable to completely eliminate the burner off cycle, and keep the burner on continuously, as long as the boiler fluid temperature is below a low temperature point signifying that the boiler fluid is not boiling.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a control system and method for periodically interrupting the fuel supply to a burner of a boiler so as to allow the liquid and foam in the boiler to settle to permit a probe-type low water cut-off sensor to sense the true liquid level in the boiler, while ensuring that the interruptions occur only when needed after the burner has been on.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention which is directed to a cycle control system for use with a boiler having a fluid therein and a method of monitoring a boiler to determine the presence of an adequate level of fluid therein. The boiler includes an associated burner and a demand control circuit such as is found in a thermostatically controlled heating system and a boiler having a temperature or pressure control for turning on and off the burner responsive to demand.
The cycle control system includes a timing circuit for producing a burner control signal having alternating on and off states. The timing circuit includes an on-period timer having an on-period that controls the on state of the burner control signal and an off-period timer having an off-period that controls the off state of the burner control signal. The off-period is of sufficient duration to allow foam and surging fluid in the boiler to settle to allow accurate determination of fluid level in the boiler. The timing circuit switches the burner control signal to the off state at the end of the on-period and switches the burner control signal to the on state at the end of the off-period. The timing circuit may be implemented in software in a microcontroller.
The timing circuit also includes a burner monitor circuit having an input for monitoring when the burner is on and off as a result of the demand control. The on-period timer is reset by the timing circuit whenever the burner monitor circuit indicates the demand control circuit has turned off the burner for a length of time sufficient to allow foam and surging fluid in the boiler to settle. This avoids prematurely turning off the burner to check the water level immediately after the burner has been off under the demand control for long enough to accomplish that check.
The cycle control system also includes a relay responsive to the burner control signal and connected to enable and disable the burner. The relay permits the demand control circuit to turn on and turn off the burner when the burner control signal is in the on state, and the relay disables power to the burner completely when the burner control signal is in the off state.
In the preferred embodiment of the invention the cycle control system includes a second relay connected in series with the first relay. The second relay disables power to the burner whenever the probe indicates the boiler fluid is low. In the most highly preferred embodiment of the invention the second relay includes a first set of contacts in series with the first relay and a second set of contacts comprising an output for controlling a fluid feeder. The first and second set of contacts work in opposition such that the second set of contacts is closed when the first set of contacts is opened.
In another aspect of the invention the timing circuit includes a low fluid level timer having a low fluid level timer period. The timing circuit starts the low fluid level timer when the probe indicates that the boiler fluid is low and the timing circuit disables power to the burner with the second relay when the probe indicates that the boiler fluid has remained low for the low fluid level timer period. This ensures that the low fluid indication from the probe is real, and not the result of a brief sloshing of fluid in the boiler.
In yet another aspect of the invention the timing circuit includes an input for connection to a low temperature sensor for monitoring the boiler fluid temperature. The cycle control system resets the on-period timer whenever the low temperature sensor indicates the boiler fluid temperature is below a desired temperature. This ensures that the heating of the boiler fluid from below the desired temperature is not interrupted. During heating of the boiler fluid from below the desired temperature, there is no risk of boiling fluid away. The cycle control system may be provided with an output for mimicking the state of the low temperature sensor switch.
To monitor whether the demand controller has kept the burner off for long enough for the probe to sense the fluid level accurately, it is preferred for the burner monitor circuit to include a hold off timer. The timing circuit starts the hold off timer when the demand control circuit turns off the burner, and the timing circuit resets the on-period timer after the hold off timer indicates the demand control circuit has kept the burner off for a length of time sufficient to allow foam and surging fluid in the boiler to settle.
The invention also includes a method of monitoring a boiler to determine the presence of an adequate level of fluid therein. The method preferably includes the steps of:
providing a probe at a predetermined level in the boiler, the probe outputting a signal to indicate the presence or absence of fluid at the probe;
providing a timing circuit having a burner control signal with alternating on and off states to disable the burner for an off-period after an on-period to permit monitoring of the true fluid level in the boiler;
turning off the burner if the probe signal indicates the absence of fluid at the probe during the off-period;
monitoring when the demand control circuit turns the burner off; and
resetting the on-period of the timing circuit after the demand control circuit has turned the burner off for a period sufficient to allow foam and surging fluid in the boiler to settle.